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Asked TI ( maker of the DRV8818 used on the gShield ) -
“Can you damaged the driver chip if you manually turn the stepper motor without power applied to the driver chip?”
TI response:
Rick Duncan replied to DRV8818: Back EMF when driver is not powered.
Hi Larry,
The answer depends on how the device is connected when spinning the motor, how fast the motor spins, and how long the motor spins.
Assuming the exposure of the device is constrained within the absolute maximum ratings, the device should not be damaged.
The overall concern is spinning the motor for long periods could overheat the device, or generate voltages outside the absolute maximum ratings.
As the motor spins any current through the body diodes can heat the device. With no power to the device, the protection circuitry is not active. The device could fail due to overtemperature conditions.
In the simple case yes. But there is more to consider than just the winding induced voltage. If you had other voltages present, oh like static electricity, it could push the voltage beyond the limit, where neither voltage alone was enough and without power the driver chip has no defenses.
It’s very similar to laser damage to eyes. you only have to exceed the limit for less than a nanosecond to fry the eye, or in this case the driver chip.
Having said all this (in this thread) I still think the odds of damage are low.
Let’s just say I don’t move my X-carve around without power applied to the chips.
How would you know? The chip manufacturers do not state a maximum for back EMF on the motor pins with or without power applied.
Rick also said it depends on how the motor is connected. Since he didn’t say specifics, it might depend on what protective circuitry is added by the board maker (none for the gShield or the X_controller with the TB6600 look alike).
The only way to know for sure would be to put a meter or scope on it and to measure what happens. I don’t think it would do any damage if it was moved a few inches, but common sense dictates that it wasn’t designed for manual movement so why chance it? Also, different sized steppers will generate different results. I’ll be using 269oz steppers and don’t plan to move them manually. Having said that I couldn’t see a 269 oz stepper causing damage IF it was moved SLOWLY for a inch or two. Bottom line there are way too many variables to know for sure, so why take chances? The only guarantee is that when they are used as designed they should work fine, any other use and you’ll be rolling the dice…
Once again I will ask - Has anyone first hand experience at causing this damage by moving by hand? I am sure many of us do yet don’t seem to have a problem. If it was a major concern wouldn’t SOMEONE have had it happen? And if no one has had it happen is it a real world problem or just a problem on paper?
The same reason most all of us take chances everyday - convenience! Probably the single thing with the highest chance of killing you is riding in a car, yet we do it everyday. Why take the chance.
Many times it’s smaller steppers and different electronics. It’s simple physics just like a loudspeaker can be used as a microphone and vice versa.
Since you are in an experimenting mood, try connecting the coil windings of your stepper to a (un-needed) speaker (no need for any power connection) and then quickly turn the stepper by hand, you’ll get some interesting “sound effects”
BTW, In the CompTIA A+ complete study guide under stepper motors (page 144) it specifically says not to move them by hand. Also, many printer companies will void the warranty if the print head is manually moved. Printers use small stepper motors.
I have read two cases on the forum where the poster swears that when they moved their gantry by hand it destroyed the gShield. A replacement gShield fixed their problem, both cases.
Now there’s no way to verify that moving the gantry caused the failure, but it was reported that way.
Even if their stories are true, that’s a pretty low failure rate.
